Conventionally, a bicycle speed change lever assembly is mounted at a portion of the bicycle frame or handlebar. Due to such an arrangement, the rider trying to perform a speed change operation needs to remove one hand from a grip portion of the handlebar while assuming a one-hand riding condition. In addition, when the rider encounters a situation which requires sudden braking or avoidance of a road projection or depression while performing a speed change operation, a certain time after the recognition of such a situation is required before actually bringing the hand to the brake or handlebar. Thus, the conventional arrangement is not considered to provide safety in performing speed change.
To solve the above problem, a speed change lever assembly has been proposed which is mounted close to a grip portion of the handlebar for enabling a speed change operation while grasping the grip portion, as disclosed in Japanese Utility Model Publication 59-24621.
However, the speed change lever assembly disclosed in the above publication has a control arm located above the handlebar, so that the thumb or forefinger grasping the grip portion must be moved to a position above the handlebar. While it is not necessary to remove all fingers from the grip portion, the need to remove the most forcefully gripping thumb or forefinger from the grip portion results in insufficient enhancement of the operability and safety of the lever assembly.
Particularly, with the so-called mountain bicycle designed to run off-road, it is desirable for the rider to use the thumb and forefinger for gripping the handlebar even at the time of performing a speed change in view of the fact that the bicycle must always run on severely rough grounds.
As a solution to this problem, there has been developed a bicycle speed change lever assembly which incorporates a bifurcated pair of operating arm portions located adjacent and under a handlebar grip portion and extending on both sides of the grip portion, as disclosed in Japanese Utility Model Publication No. 62-38878 for example. According to the arrangement of this publication, the rider extends the thumb and forefinger downwardly with the handlebar grip portion held therebetween, and the bifurcated pair of arm portions may be pivoted horizontally by the extended thumb and forefinger. Thus, the thumb and forefinger need not be brought out of gripping engagement with the grip portion at the time of performing a speed change operation, thereby greatly improving the safety.
However, with the speed change lever assembly of the latter publication, it is impossible to provide a large pivotal angle for the operating arm portions. Thus, the lever assembly is problematic in the inability of providing five to eight speed stages which are normally required for sports-type bicycles.
More specifically, with the lever assembly of the latter publication, the bifurcated pair of operating arm portions are located on both sides of the handlebar, and the rider gripping the handlebar extends the thumb and forefinger downwardly to pivot the arm portions within the range defined between the thumb and forefinger. Therefore, the pivotal range for the arm portions is limited by the thumb and forefinger extended from the handlebar. As a result, the pivotal range f or the arm portions becomes small, thus failing to provide a required number of speed stages for a normal sports-type bicycle.
Further, the operating arm portions of the lever assembly disclosed in the latter publication are pivotally supported on a lever shaft which is perpendicularly mounted to the handlebar. Thus, the arm portions are pivoted in a plane which is parallel to the handlebar or grip portion.
However, the thumb and forefinger are different in length and movable range, so that the optimum pivotal path of the operating arm portions for the thumb does not coincide with that for the forefinger. Thus, the operating arm portions, if optimized in pivotal path for one of the thumb and forefinger, cannot be conveniently operated by the other.
Further, the operating arm portions need be relatively long to effectively utilize the movable range of the thumb and forefinger and to increase the pivotal range of the arm portions. However, since the respective arm portions are designed to pivot in the same plane, they interfere with the fingers grasping the grip portion during a speed change operation, thereby hindering the speed change operation. Due to this, the pivotal range of the operating arm portions is additionally limited, which is an obstacle in increasing the number of speed stages.